1. Field of the Invention
The present invention relates to a pulse tube refrigerator, particularly to a pulse tube refrigerator which permits improving the efficiency.
2. Description of the Related Art
A pulse tube refrigerator is known to the art as a refrigerator which is relatively simple in construction and which permits reaching a relatively low temperature. Various types of pulse tube refrigerators of this kind are known to the art. Any type of the pulse tube refrigerator basically comprises a coldness generator consisting of a regenerator and a pulse tube connected in series to the regenerator and is constructed such that a coolant gas of a high pressure is introduced through the regenerator into the pulse tube and, then, discharged to the outside via the reverse passageway. During the process of the discharge to the outside, the high pressure coolant gas is expanded so as to generate coldness within the pulse tube.
In order to increase the coldness generation in a pulse tube refrigerator of this type, it is necessary to provide a difference between the phase in the pressure fluctuation within the pulse tube and the phase in the displacement of the coolant gas. Because of the particular requirement, a system for providing such a phase difference is provided in many cases in the conventional pulse tube refrigerator.
FIG. 1 shows a conventional pulse tube refrigerator comprising a system for providing a phase difference. A reference numeral 1 in FIG. 1 denotes a coldness generator, with a rotary gas compressor being denoted by a reference numeral 2. As shown in the drawing, the coldness generator 1 comprises a regenerator 3 and a pulse tube 5 connected in series to the regenerator 3 with a low temperature heat exchanger 4 interposed therebetween. The regenerator 3 comprises a vessel 6 made of a heat insulating material or a metallic material having a low thermal conductivity and a refrigerant 7 housed in the vessel 6. The refrigerant 7 is formed of, for example, a stainless steel mesh or a copper mesh. On the other hand, the pulse tube 5 is formed into a pipe and is made of a heat insulating material or a metallic material having a low thermal conductivity.
An inlet 8 of the regenerator 3 is connected to a discharge passageway 11 and to a suction passageway 12 of a gas compressor 2 via a high pressure valve 9 and a low pressure valve 10, respectively. These high pressure valve 9 and low pressure valve 10 are alternately allowed to be opened or closed periodically by a valve controller (not shown). To be more specific, these valves 9 and 10 are controlled such that, when one of these valves is opened, the other valve is closed, and vice versa.
On the other hand, a pipe 13 for a so-called double inlet passageway (hereinafter referred to as "double inlet line 13") is provided between one end (or upper end in the drawing) of the pulse tube 5 and the inlet 8 of the regenerator 3. A valve 14 for controlling the coolant gas flow rate is disposed midway of the double inlet line 13. Said one end portion of the pulse tube 5 is also connected to a buffer tank 16 via an orifice valve 15. A coolant gas such as a helium gas is sealed with a predetermined pressure within the system described above.
In the conventional pulse tube refrigerator of the construction described above, a pressure fluctuation is generated within the pulse tube 5 by the alternate opening/closing of the high pressure valve 9 and the low pressure valve 10. What should be noted is that coldness is generated within the pulse tube 5 by providing a difference in phase between the pressure fluctuation and the displacement of the coolant gas. The coldness thus generated partly serves to cool an object to be cooled via the low temperature heat exchanger. The remainder of the coldness is subjected to cooling of the refrigerant when the coolant gas flows via the reverse passageway.
In the system described above, it is possible to provide an optimum operating condition by controlling the degree of opening of each of the valve 14 and the orifice valve 15. In other words, the double inlet line 13, the valve 14, the orifice valve 15 and the buffer tank 16 collectively serve to form the phase difference referred to above.
FIG. 2 shows another conventional pulse tube refrigerator. Used in this refrigerator is a reciprocating gas compressor. To be more specific, a gas compressor 2b connected to the inlet 8 of the regenerator 3 is of a reciprocating type, which comprises a compression chamber 18 defined by a cylinder 17a and a piston 17b. One end of a piston rod 17c is connected to the back surface of the piston 17b, with the other end being guided by a guide mechanism 19 so as to be joined to a reciprocating driving source (not shown).
In the conventional pulse tube refrigerator shown in FIG. 2, the piston 17b is moved upward in the drawing so as to diminish the inner volume of the compression chamber 18, with the result that the compressed gas flows partly through the regenerator 3 into the pulse tube 5 and partly through the pipe 14 into the pulse tube 5 and into the buffer tank 16. Then, when the piston 17b is moved downward, the coolant gas within the pulse tube 5 flows partly through the refrigerator 3 into the compression chamber 18 and partly through the double inlet line 13 into the compression chamber 18. The particular flow of the coolant gas brings about a pressure fluctuation within the pulse tube 5 so as to generate coldness. The coldness thus generated partly serves to cool the object to be cooled via the low temperature heat exchanger 4. The remainder of the coldness permits the refrigerant 7 to be cooled when the coolant gas flows through the reverse passageway. It should be noted that an optimum operating condition can be provided by controlling the degree of opening of each of the valve 14 and the orifice valve 15. In other words, the double inlet line 13, the valve 14, the orifice valve 15 and the buffer tank 16 collectively serve to form the phase difference referred to previously.
However, the conventional pulse tube refrigerators shown in FIGS. 1 and 2 give rise to serious problems. When it comes to the conventional pulse tube refrigerator shown in FIG. 1, the range of control of the phase difference noted previously is restricted by the opening/closing operation of each of the high pressure valve 9 and the low pressure valve 10. On the other hand, when it comes to the conventional pulse tube refrigerator shown in FIG. 2, the range of control of the phase difference is restricted by the gas compressor 2. In short, it is difficult to provide a sufficiently large phase difference in any of the conventional pulse tube refrigerators shown in FIGS. 1 and 2. It follows that the conventional pulse tube refrigerators shown in these drawings is lower in efficiency than a Starling refrigerator which comprises an expansion piston disposed in a low temperature portion, said expansion piston serving to forcedly provide a desired phase difference.
As described above, the conventional pulse tube refrigerator is advantageous in that a movable member need not be disposed in a low temperature portion, but leaves room for further improvement in terms of efficiency.
What should also be noted is that, in the conventional refrigerator shown in FIG. 2, the frictional resistance generated between the cylinder 17a and the piston 17b causes reduction in the compression efficiency, leading to reduction in the efficiency of the refrigerator.